Radiation detector array using radiation sensitive bridges

ABSTRACT

An infrared (IR) simulator is disclosed in which an array of pixels is defined on an insulative substrate by resistor bridges which contact the substrate at spaced locations and are separated from the substrate, and thereby thermally insulated therefrom, between the contact locations. Semiconductor drive circuits on the substrate enable desired current flows through the resistor bridges in response to input control signals, thereby establishing the appropriate IR radiation from each of the pixels. The drive circuits and also at least some of the electrical lead lines are preferably located under the resistor bridges. A thermal reflector below each bridge shields the drive circuit and reflects radiation to enhance the IR output. The drive circuits employ sample and hold circuits which produce a substantially flicker-free operation, with the resistor bridges being impedance matched with their respective drive circuits. The resistor bridges may be formed by coating insulative base bridges with a resistive layer having the desired properties, and overcoating the resistive layers with a thermally emissive material. The array is preferably formed on a silicon-on-sapphire (SOS) wafer. Arrays of electromagnetic radiation bridge detectors may also be formed, with the bridges having either resistor, thermocouple or Schottky junction constructions.

This is a division of application Ser. No. 07/370,109, filed on June 21,1989, which was a file wrapper CIP application of 07/228,630, filed Aug.4, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to systems for simulating an infrared image foruse in testing infrared seekers. It also relates to detector arrays forelectromagnetic radiation including infrared and microwave.

2. Description of the Related Art

It would be highly desirable to be able to simulate a real-time infrared(IR--roughly 0.7-20 microns wavelength) image that is substantially freeof flicker. This would provide an effective way to test IR detectors,also referred to as "seekers" and "focal plane arrays". At present,problems of excessive flicker impose a serious constraint on IRsimulation systems. A basic problem with image flicker is that itcreates a false target indication, since flicker corresponds to a changein the temperature of the IR image. Unlike the human eye whichintegrates light flicker over a period of about 30-50 msec., IRdetectors integrate flickers over periods of only about 1-5 msec. Thus,there is a significant range over which flicker (in the visiblespectrum) would not be detected by the human eye but would be picked upby an IR detector if it is within the IR spectrum.

Excessive flicker has been avoided heretofore with the use of a Bly cellto project a static image that has been applied to the cell. Bly cellsare described in Vincent T. Bly, "Passive Visible to Infrared Transducerfor Dynamic Infrared Image Simulation", Optical Engineering, Nov./Dec.1982, Vol. 21, No. 6, pp. 1079-1082. However, the requirement that thistype of system be operated with a static image is a significantlimitation, since a more meaningful test of IR detectors calls for thedetection of images that can change in real-time.

A prior attempt to produce an IR simulation system with a real-timeimage involved the formation of a video image by a cathode ray tube(CRT). The CRT video image was applied as an input to a liquid crystallight valve (LCLV), to which an IR readout beam was applied. The LCLVmodulated the IR readout beam with the video image from the CRT toproduce a corresponding IR video image. See S. T. Wu et al, "InfraredLiquid Crystal Light Valve", Proceedings of the SPIE, Vol. 572, pages94-101, August, 1985.

This approach unfortunately was found to result in a substantial amountof flicker. The problem is that the illuminated pixels on the CRT screendecay in intensity over time prior to the next electron beam scan. Thiscauses an undesirable intensity gradient to appear on a projected IRimage from an IR-LCLV which is coupled to the CRT, and an IR detectorwill then detect a non-uniform image. Because the detector is generallylooking for intensity gradients, or edges, by which its associatedalgorithms determine the presence of "targets", such intensity gradientsare misleading. While this problem could theoretically be solved bysynchronizing the IR detector scan with the CRT electron beam scan, suchsynchronization may not be desired in many applications. Thus, althoughan IR-LCLV has the capability of projecting high resolution, highdynamic range, real-time simulated IR images when compared to a Blycell, this advantage is mitigated by the CRT pixel decay. Furthermore,electrically driven matrix emitter devices have flicker if driven withsimple RC-type pixel addressing circuits, since the RC decay is similarin effect to the phosphor decay of the CRT.

Modifications of the basic CRT-LCLV system described above might beenvisioned to reduce or eliminate flicker, but they introduce otherproblems. In one such modification, two storage CRTs are provided withshutters in front of each screen. Operation is alternated between thetwo CRTs by means of the shutters, so that they are alternately appliedto the LCLV. By staggering the video data frames between the two CRTs,the phosphor decay seen by the LCLV could theoretically be reducedsignificantly. However, in such a system, it may be difficult toimplement the very fast shutter coordination that would be necessary tosubstantially avoid flicker. Furthermore, storage CRTs are non-uniform,resulting in image differences and consequent flicker.

Another approach would be to use a single CRT, but to increase the framerate of the Raster scan from the conventional rate of about 30 Hz to amuch higher rate, perhaps about 1,000 Hz. The CRTs of the future mayprovide higher bandwidths than that presently attainable, thereby makingthis approach more attractive.

A possible approach which does not provide real-time addressable imagesis the use of a "flicker-free" film or slide projector like theSCANAGON® device produced by Robert Woltz Associates, Inc. of NewportBeach, Calif. and disclosed in U.S. Pat. Nos. 4,113,367 and 4,126,386,or a comparable image projector. While the potential may exist for thislimited technique, it has not been demonstrated to provide jitter-freeand flicker-free images. Furthermore, this method will not providereal-time electronically updatable imagery.

In addition to flicker-free images, it is very important for an IRsimulation system to achieve high spatial resolution, large dynamictemperature ranges and fast response. Spatial resolutions should be atleast 500×500 pixels, with flicker less than 1%. For some applicationsframe rates should be 100 Hz or greater, and the dynamic thermal rangeshould ideally be from near room temperature (some applications requirecooled background temperatures) to 1,000° C., particularly in the 3-5micron spectral range. This combination of dynamic range and responsetime requirements is difficult for commercially available liquidcrystals to achieve. Furthermore, most IR simulation applicationsrequire the simulator to be mounted on the two-axis target arm of aCarco table, which is then moved relative to the IR seeker being tested.For this purpose weight and size limitations are very important. Theliquid crystal based IR simulator requires a large blackbody source,typically in excess of 100 pounds, and an expensive wire grid polarizerto be mounted with the active matrix. This reduces the mobility of thesupporting Carco table arm.

Liquid crystal systems are also limited in dynamic range because it isdifficult to photogenerate enough charge in the silicon substrate, whilemaintaining spatial resolution, to rotate the liquid crystal moleculescompletely. Furthermore, there is a restriction on the speed of responseof liquid crystals if a reasonable thermal dynamic range is to bemaintained. The flicker problem associated with liquid crystal systemscan be reduced by utilizing a flicker-free visible addressing source forthe liquid crystal light valve. However, known sources of this type arelimited in speed, resolution, weight and size.

U.S. Pat. No. 4,724,356 to Daehler discloses a resistorbased IRsimulator in which an array of resistors are individually addressed tostimulate IR emission. The Daehler approach is also discussed inDaehler, "Infrared Display Array", SPIE Vol. 765, Imaging Sensors andDisplays (1987), pages 94-101 and Burriesci et al., "A Dynamic RAMImaging Display Technology Utilizing Silicon Blackbody Emitters", SPIEVol. 765, Imaging Sensors and Displays (1987), pages 112-122. Both theresistors and their respective drive transistor circuits are formed froma bulk silicon wafer. An air gap groove is formed under the resistors tohelp reduce thermal conduction losses to the underlying bulk siliconwafer. To facilitate the selective etching used to form the insulatingair gap grooves, the top layer of the wafer is heavily doped. However,this doping makes the top layer electrically conductive, resulting in asignificant impedance mismatch between the resistors and theirrespective transistor drive circuits. As a result, most of the inputpower is lost in the drive circuitry, rather than going into IRradiation. This reduction in the power transfer to the resistors resultsin a need for even more input power to the transistors, leading to apotential overheating problem.

In addition to an inefficient use of input power, in the Daehlerapproach only a relatively small portion of the pixel area (less than10%) is actually occupied by the radiating resistor. Much of the areathat might otherwise be devoted to the resistor is occupied by the drivetransistors and the insulating air gap. As a result, to achieve a givenapparent temperature for the pixel as a whole the resistor itself mustbe heated to a significantly higher temperature. This substantiallyreduces the system's effective thermal dynamic range.

Another problem with the Daehler approach relates to the electrical leadlines used to address the individual pixels. Relatively large siliconwafers must be used with present technology to achieve a significantnumber of pixels. For example, a wafer of about four inches is requiredfor a 512×512 array. This means that the electrical lead lines on thechip must also be about four inches long. This in turn leads to twosignificant problems. First, there is a large amount of capacitivecoupling associated with the long lead lines. Parasitic capacitancebetween conductive substrate and metal lead lines can limit the responsetime. Second, defects such as pin holes can appear in the oxideinsulator formed on the substrate, causing a short circuit between theline and the silicon substrate which renders the associated pixel orpixels inoperative. The longer the lines, the greater is the probabilityof such defects. Daehler tries to resolve this problem by limiting thesize of the array to 8×128 pixels, and coupling many such arraystogether side-by-side to produce an aggregate array of reasonable size.However, the coupling process itself introduces significantcomplexities.

There is also a need for a two-dimensional IR detector array, and fordetector arrays at other electromagnetic radiation (emr) wavelengthssuch as microwave, which have a fast response and make efficient use ofavailable area.

Various types of IR thermal detectors are currently available. Inpyroelectric detectors, a temperature change alters the dipole moment ofthe material, resulting in a charge difference between crystal faces.This type of detector is discussed in an article by Watton,"Ferroelectrics for Infrared Detection and Imaging", Proceedings of theSixth IEEE International Symposium on the Application of Ferroelectrics,June 1986, Bethlehem, Pa. pages 172-181. Another type of IR detector isthe bolometer, in which the energy of absorbed radiation raises thetemperature of the detecting element to change its electricalresistance, which change is measured as an indication of the amount ofreceived radiation. An array of such devices is disclosed in Boninsegniet al., "Low Temperature Bolometer Array", Review of ScientificInstruments, Vol. 60, No. 4, April 1989, pages 661-665. Both thepyroelectric and bolometer devices exhibit a relatively slow rate ofresponse to changes in IR level, and also require relatively largeareas. The pyroelectric thermal detector also suffers from limitedresolvable temperature differences, low yield and high cost processing,a necessary hybrid integration and AC operation with a chopper. Thelimited temperature resolution, which is principally due to the inherentthermal conduction loss and large thermal mass, can be improved bythinning and reticulating the pyroelectric wafer, but this involves verydifficult and expensive processing.

Another approach to IR detection involves an array of single crystalSchottky junctions, as disclosed in Tsaur et al., "IR SiSchottky-Barrier Infrared Detectors With 10-μm Cutoff Wavelength", IEEEElectron Device Letters, Vol. 9, No. 12, December 1988, pages 650-653.This device is used as a photon detector rather than a thermal detector,has a very limited spectral range, and has to be operated at cryogenictemperatures.

Thermocouple devices provide another IR thermal detector. In this typeof device junctions are formed between dissimilar materials, with avoltage induced across the junction in response to heating. Such adevice is disclosed in Lahiji et al., "A Batch-Fabricated SiliconThermopile Infrared Detector", IEEE Transactions on Electron Devices,Jan. 1982, pages 14-22. In this article a series of thermocouples havehot junctions which are supported on a thin silicon membrane byintegrated circuit techniques. The membrane area is coated with a layerof IR absorbing material to efficiently absorb energy from the visibleto the far infrared. The sensitivity of the device is limited becausethe thermocouples are formed directly on the substrate and thereforecannot heat as much as desired due to the thermal conduction losses.Also, a relatively large area is required for the detector.

Bolometers and crystal detectors have also been used to sense emr atmicrowave frequencies. Since relatively large sensor dimensions arenecessary, single detectors have been used to scan an incoming image,rather than using a detector array to sense the entire image at onetime. The result has been slow frame rates. The same problem hasattended the use of single scanning junction detectors in the IRrequire.

SUMMARY OF THE INVENTION

In view of the above problems, the goal of the present invention is toprovide an IR simulation system which is substantially flicker-free,light in weight, has a large thermal dynamic range and fast response, ispower efficient, devotes most of the pixel area to the radiatingelement, and avoids the capacitive coupling and insulator defectproblems associated with long leas lines on a single crystalsemiconductor substrate. A two-dimensional emr detector array withsimilar characteristics is also included.

This goal is achieved in accordance with the invention by providing anIR pixel matrix in the form of an array of resistor bridges formed on aninsulating substrate, such as sapphire. The resistor bridges span aportion of each pixel cell, contact the substrate at spaced-apartcontact locations, and are shaped to form a thermally insulative gapbetween the bridge and substrate. A semiconductor drive circuit on thesubstrate enables a desired amount of current flow through the bridge inresponse to an input control signal to heat the bridge, therebyproducing a desired amount of IR radiation. Control, actuating and powerlead lines extend along the substrate to deliver control and actuatingsignals to the drive circuits, and an electrical power signal to theresistor bridges; since the substrate is insulative, capacitive couplingand shorting problems are mitigated even with long lead lines.

In the preferred embodiment the drive circuits are located on thesubstrate surface under the resistor bridges, and at least some of thelead lines also extend under the bridges. Each pixel cell includes athermally reflective element between the resistor bridge and drivecircuit to reflect IR radiation away from the drive circuit. Thethermally reflective element may comprise either a second bridge formedfrom a thermally reflective material which bridges the drive circuit andis spaced from both that circuit and the resistor bridge, or a thermallyreflective layer which overlies an insulative layer on the drivecircuit.

To reduce flicker to less than 1%, the drive circuit for each pixelcomprises a sample and hold circuit in which a first transistor appliesa control signal to a holding capacitor, and a second transistor appliesa power signal to the resistor bridge in response to the capacitorsignal. The electrical impedance of the resistor bridge preferablymatches that of the drive circuit.

The resistor bridges may be implemented in various ways. In oneimplementation they comprise a base bridge formed from an insulativematerial, with a resistive layer over the base. The resistivity andthickness of the resistive layer are selected to produce a desiredresistance. The resistive layer may itself be surmounted by a coating ofhigh thermal emissive material. The resistor bridge may also be formedfrom a material having an enhanced porosity which decreases its thermalconductivity. To concentrate the voltage drops towards the centers ofthe bridges and away from their connections with the substrate, thebridges may be tapered so that their cross-sectional areas near thecenter are significantly less than near their contacts with thesubstrate.

The resulting IR pixel array has a significantly better combination ofthermal dynamic range, speed of response, resolution, weight, size,efficiency and flicker-free operation than any other known system.

The invention also includes two-dimensional emr sensing arraysconsisting of an array of emr detector cells formed on a substrate. Eachcell includes an emr sensitive bridge structure. The shape of the bridgestructure is designed to form a generally insulative gap between thebridge and the substrate. The bridge structure has a definedcharacteristic which varies with the amount of received emr, and meansare provided for monitoring this characteristic for each of the cells todetermine the level of emr incident on each cell.

The bridge structures can be implemented by resistor bridges,thermocouple junction bridges, or Schottky junction bridges. Withresistor bridges, various bridge geometries may be selected to reducethe area of the bridge support legs relative to the center span, andthereby assist in thermally insulating the center span from thesubstrate. Multiple-stage amplifier readout circuits are preferablyemployed in connection with output voltage divider circuits to monitorthe effective bridge resistances with a reduced Johnson noise. Thinlayers of conductive material may be added to the support legs toelectrically connect the center span to the underlying monitoringcircuits, the conductive material layers being thin enough tosubstantially maintain the thermal isolation of the bridge center spansfrom the substrate. The resistor bridges are preferably formed from anamorphous semiconductor material.

With a thermocouple bridge detector array, the thermocouples preferablycomprise a stacked plurality of alternating semiconductor and metallayers. The junction devices are connected in series with resistors andoutput voltage divider circuits, with the voltages across the outputresistors monitored as a function of the emr received by thethermocouple bridges.

The Schottky junction bridge embodiment employs a bridge configured asadjacent layers of a semiconductor material and a metal or dopedsemiconductor, meeting along a Schottky contact junction. Thesemiconductor is preferably amorphous germanium or amorphous tin.

The detector bridges are preferably coated with a layer of emr absorbingmaterial to increase their sensitivity. Readout circuits for eachdetector cell are preferably located on the substrate at least partiallyunder their respective bridges to conserve substrate area. VArious leadlines may also extend under the bridge structures to further conservesubstrate area. A lens is employed to image an emr source onto thetwo-dimensional array.

Additional features and advantages of the invention will be apparent tothose skilled in the art from the following detailed description ofpreferred embodiments, taken together with the accompanying drawings, inwhich:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an IR simulator system which incorporatesthe present invention;

FIG. 2 is a diagrammatic plan view of a portion of the pixel cell arrayused to create an IR image;

FIG. 3 is a schematic diagram of the drive circuitry employed in eachpixel cell;

FIGS. 4-6 are sectional views showing different embodiments of theresistor bridge which provides the IR radiating element for each pixel;

FIGS. 7-9 are sectional views illustrating various embodiments of therelationship between the resistor bridge of each pixel and the drivecircuitry therefore;

FIG. 10 is an illustrative plan view showing the disposition ofelectrical lead lines with respect to the resistor bridge for oneembodiment.

FIG. 11 is a sectional view of a resistor bridge emr detector andreadout circuit in accordance with the invention;

FIGS. 12(a), 13(a) and 14(a) are plan views and FIGS. 12(b), 13(b) and14(b) are front elevation views of three different resistor bridgeconfigurations which thermally isolate the center span of the bridgefrom the underlying substrate;

FIG. 15 is a simplified schematic diagram of a readout circuit for aresistor bridge emr detector;

FIG. 16 is a more complete schematic diagram of the circuit shown inFIG. 15;

FIG. 17 is a sectional view showing the formation of a transistor andcapacitor of FIG. 16 in a single monolithic construction;

FIG. 18 is a schematic diagram of another readout circuit for a bridgeresistor emr detector which reduces Johnson noise;

FIG. 19 is a sectional view of a thermocouple bridge emr detector;

FIG. 20 is a simplified schematic diagram of a readout circuit for thedetector of FIG. 19;

FIG. 21 is a sectional view of a Schottky junction bridge emr detector;

FIG. 22 is a simplified schematic diagram of a readout circuit for theemr detector of FIG. 21; and

FIG. 23 is a block diagram showing the imaging of an emr source onto adetector array constructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the basic elements of an IR simulation system constructedin accordance with the invention. An IR radiating mechanism 2 is placedwithin a vacuum chamber 4 which has an IR window 6 at its forward end. Avacuum pump 8 evacuates the chamber in a conventional manner. The IRstructure includes an insulative substrate 10, preferably sapphire, atwo-dimensional array 12 of IR emitting elements on the forward side ofsubstrate 10, and a cooling mechanism 14 contacting the rear of thesubstrate. The cooling element is preferably a copper block cooled bywater or freon flowing through inlet and outlet pipes 16. The IR window6 is formed from a suitable material such germanium or zinc selinide,and is preferably provided with an anti-reflective coating to preventreflection cf emitted IR radiation back into the chamber.

The individual IR emitting elements within the array 12 providecontrolled emissions which are directed by a lens 18 onto an IR detector20 being tested. The emissions from each individual IR radiating elementare controlled by electrical signals applied to the array. Theelectrical leads into the array include a power line 22 connected to aDC power source, a series of data lines 24 which carry image informationsignals to the various IR elements, a series of strobe lines 26 whichstrobe rows of IR elements in succession in coordination with the imagesignal, and a ground line 28. The image data input signals may representan actual video image, a computer simulation, or other desired pattern.

The IR radiating array is divided into a matrix of pixel cells, each ofwhich is individually controlled to emit a desired amount of IRradiation. The resolution of the IR image is governed by the number ofpixels, which are typically provided in a 256×256, 512×512 or 1024×1024array. A section of such an array is illustrated in FIG. 2. Each pixelcell includes a resistor bridge 30, which is the actual IR radiatingelement, and a drive circuit 32 for the resistor bridge. Each pixel cellis typically about 150 microns square, while the resistor bridges areabout 120 microns square. The drive circuits employ two transistors,each of which is typically about 50 microns square. The aluminumelectrical lead lines are typically about 25 microns wide.

FIG. 2 is a simplified drawing which illustrates the functionalrelationship between the various elements in the array, but does notshow them in their preferred physical arrangement; this will bediscussed further below. One end of each resistor bridge 30 is connectedto a DC power line 22, while the other end is connected to itsassociated drive circuit 32. Each drive circuit acts in response to anapplied image signal from line 24 to control the flow of current fromthe power line, through the resistor bridge and drive circuit to ground.

The pixel cells are arranged in a matrix of rows and columns. Electricalinputs are provided in a manner that allows each pixel to be separatelyprogrammed, yet avoids the need for separate lead lines for each pixel.Discrete signals are applied to the line 24 for each column, so thateach drive circuit in a given column receives the same signal, but thesignals vary from column-to-column. Each row of pixel cells is strobedin succession via strobe lines 26, which are connected to the pixeldrive circuits. The application of a strobe signal enables the operationof a drive circuit, which otherwise holds off the flow of any currentthrough the resistor bridge. By varying the pattern of image datasignals in synchronism with the progression of the strobe signal fromrow-to-row, a unique signal can be applied to each pixel cell toestablish the resistor current through that cell at a unique value. Thedrive circuits are designed so that they can acquire an image datasignal only during the brief period when the circuit is being strobed,so that the changing pattern of signals while other rows are beingstrobed has no effect upon the signal acquired by a particular pixel.Each drive circuit includes a sample and hold circuit which holds theacquired image data signal during the period of time required to strobeall of the other rows. With a frame rate in the order of 100 Hz, thesample and hold circuits keep the decay of the video signals acquired bytheir respective pixels to less than 1%. This results in a substantiallyflicker-free operation.

A schematic diagram of the preferred drive circuit for each pixel isgiven in FIG. 3. The circuit is preferably implemented with asilicon-on-sapphire (SOS) wafer in which the active circuit elements areformed from a silicon layer 12 on the sapphire substrate 10. SOS wafersare currently available in 5 inch diameters, and larger wafers willprobably be produced in the future. Since an insulating substrate isemployed, the prior problem of shorting and capacitive coupling from thelead lines is substantially eliminated, thereby permitting the use oflarge arrays.

Each sample and hold circuit includes a first FET T1 with itssource-drain circuit connecting the image data input to a holdingcapacitor C1. The signal held by C1 is applied to the gate of a secondFET T2, and controls the amount of current flow permitted by T2. Thebridge resistor 30 is connected between the DC power source and thedrain or source of T2, depending upon the transistor connection; theopposite source-drain terminal of T2 is connected to ground. Thus, theamount of current permitted to flow through resistor 30, and accordinglythe amount of IR radiation emitted by resistor 30, is controlled by thevideo signal held by C1 and applied to the gate of T2.

A simple implementation of bridge resistor 30 is shown in FIG. 4. Theresistor has a center span 34 that is elevated above the surface ofsapphire substrate 10 by a pair of legs 36 at opposite ends of thecenter span. The vertical air gap clearance between center span 34 andthe substrate is preferably about 2-5 microns, which is sufficient toprevent substantial thermal conduction loss from the resistor to thesubstrate. The method of forming such a bridge resistor is known, and isdescribed in U.S. Pat. No. 4,239,312 to Myer et al., assigned to HughesAircraft Company, the assignee of the present invention. The bridgeresistor could also be implemented with electrically conductive,metallic vertical legs and a planar center span of resistor material. Inthis configuration, because the legs have high electrical conductivity,there is a proportionate reduction in the voltage-induced thermalgradients, and reduction in stress.

Whereas the resistors employed in the Daehler IR simulator were limitedto boron doped silicon because of the bulk silicon substrate, with thepresent invention the resistor material can be selected to optimize itscharacteristics. Ideally, the resistor should have a low thermalconductivity to limit heat transfer to the substrate, an electricalconductivity which matches the resistor impedance to the impedance ofthe drive circuit, a low coefficient of thermal expansion to prevent theresistor from expanding significantly with respect to the lowertemperature substrate, and a high melting point. One suitable resistormaterial is polysilicon, which has a high melting temperature,controllable resistivity, small temperature dependent resistivity, and alow coefficient of thermal expansion. Other materials which may besuitable include graphite, which has good temperature properties, highemissivity, titanium oxides, tantalum oxides, silicon oxides andcermets.

To minimize thermal stress at the junction between the bridge resistorand the substrate, the bridge may be fabricated as shown in FIG. 5 sothat its legs 36 are relatively thick where they contact the substrate,and progressively taper in cross-sectional area towards the center span34. This arrangement reduces the resistance of the legs, particularly inthe vicinity of the substrate, and also adds mechanical support.Accordingly, the bulk of the voltage drop and the resultant hightemperature will occur at the center span 34, which is desirable to bothreduce mechanical stress and enhance IR radiation. The temperature ofthe bridge will be lower along its legs, particularly near thesubstrate, since less power is dissipated there.

Further protection against heat loss may be obtained by increasing theporosity of the resistor material through electron beam deposition orother suitable technique. An increased porosity reduces the thermalconductivity of the material, making it even less susceptible.

In another embodiment, illustrated in FIG. 6, the resistor bridge isformed from a base bridge 38 of insulative material such as Si0₂, with athin resistive layer 40 over the base bridge 38. The resistivity andthickness of the resistive layer 40 are selected to produce the desiredresistance for the bridge. A very thin metal or ceramicmetal (cermet)layer, preferably on the order of about 50 Angstroms or 0.005 micronthick, would be suitable. The insulative base bridge 38 should itself bemade as thin as possible, consistent with the required mechanicalsupport, to provide fast response and low thermal conductivity. Si0₂about 0.5-1 micron thick would be suitable. The resistive layer 40 mightbe coated with a layer of high thermal emissive material 42, such ascarbon black or gold black, to improve the radiative efficiency of thebridge. The thermally emissive layer may then be sealed with a sealingcoat of Si0₂ about 50-100 or 0.005-0.01 Angstroms thick.

To preserve space on the substrate and free up more area for theresistor bridges, the drive circuits 32 are preferably located under thebridges, as illustrated in FIG. 7. To maintain the drive circuitry atreasonable operating temperatures, it may be shielded from the heat ofthe resistor by means of a second bridge 44 as illustrated in FIG. 8.This second bridge is spaced from both the resistor bridge 30 and thedrive circuit 32, and is formed from a thermally reflective materialsuch as aluminum. In addition to protecting the circuit elements fromoverheating, the reflective bridge 44 increases the pixel output power,since most of the IR radiation which it reflects is transmitted throughthe thin resistor bridge to enhance the output IR radiation.

FIG. 9 illustrates an alternate to the discrete reflective bridge 44. Inthis embodiment an electrically insulative layer 46 such as Si0₂ is laiddown over the drive circuit 32, followed by a thermally reflective layer48 such as aluminum over the insulative layer.

Additional area on the array may be conserved and devoted to theradiating resistor elements by directing at least some of the lead linesunder the bridge resistors. Such an arrangement is illustrated in FIG.10. In this single layer metallization scheme, the image data, power andground lines for each pixel 50 are run under the resistor bridge 30,while the strobe line extends along the side of the resistor. With adouble layer metallization, all four lines could be separated byinsulating layers and run under the bridge. Single and double layermetallization techniques are well known in the art. For further savingsin real estate, adjacent pixels could share the same power and groundlines.

The preferred technique for fabricating the array is to first fabricatethe drive circuits on an SOS wafer. The whole substrate is then coveredwith a spacer layer, such as potassium chloride or aluminum, which canbe selectively etched. A resist layer is then laid on top of the spacerlayer to define the pixels. The resist at the intended resistor bridgelocations is next exposed, the unexposed resist is washed off, and thespacer layer below the removed resist is etched away. After removing theremaining resist the bridge material is overcoated on the remainingspacer layer by a technique such as sputtering or electron beamevaporation, depending upon the bridge material and properties desired.A shadow mask technique might also be employed to form the bridges, butit is difficult to obtain desirably high pixel density and small bridgedimension with this technique. Finally, the remaining spacer material isremoved by standard techniques, such as dissolving away potassiumchloride or etching away aluminum.

With the described IR simulation system, 60-65% of the pixel area can bedevoted to the resistor. As a result, the resistors do not need to beheated to as high a temperature to attain a given effective pixeltemperature as with the prior system that employed much smallerresistors. The resistors can be tailored to impedance match the drivecircuits, thereby significantly improving the efficiency of the device.The problems of capacitive and electrical shorting associated with thelead lines is also substantially alleviated. The device is calculated tohave an effective thermal dynamic range of 1,000° C. in the 3-5 micronregion, and 600°-700° C. in the 8-12 micron region (with the substrateat room temperature). Frame rates of 200 Hz are achievable withoutbothersome flicker.

The bridge structure of the present invention can also be used as asensitive and space-efficient emr detector for IR, microwave and otherwavelengths. A particular range of wavelengths can be selected forsensing by an appropriate selection of the bridge dimensions, generallyfollowing the rule that the length of the bridge should be at least halfthe desired wavelength. IR wavelengths are quite small, so the bridgesin this range will generally be considerably larger than a halfwavelength. Microwave wavelengths are much longer, on the order of cms.,so microwave detector bridges would not need to be much bigger than ahalf wavelength. With the use of a refractive lens or, in the case ofmicrowaves, a focusing reflector, the emr beam can be at least partiallyfocused to permit the use of smaller detector bridges.

FIG. 11 illustrates a resistance bridge detector that can be used forthis purpose. The bridge 52 consists of a center span 54 with supportlegs 56 at either end extending up from a substrate 57, and a readoutcircuit 58 on the substrate under the chip and at least partially shadedthereby from incident emr.

A preferred material for the bridge is amorphous silicon, which has arelatively high temperature coefficient of resistance, is compatiblewith integrated circuit processing, and can be formed with controlledmechanical stresses. Other amorphous semiconductors are also suitable,such as Ge, SiGe, and SiC. Materials which are generally unsuitable forthe resistor bridge include metals because of high thermal conductivityand a low electrical resistance and low temperature coefficient ofresistance, insulators because of low electrical conductivity, andcermets because of low temperature coefficients of resistance. Thesubstrate 57 is preferably single crystal silicon for relatively smalldimensions in the IR regime, or a silicon-on-insulator construction forlarger microwave dimensions.

A layer 60 of an emr absorbent material, such as metallic black or blackpaint for IR, is provided on the bridge center span, which is the emrtarget area, to increase the emr-induced temperature variation. Variousgeometric configurations for the bridge can also be designed to assistin thermally isolating the center span from the substrate, and therebyincrease the potential degree of heating for the center span and thusthe device's sensitivity. This goal may be accomplished by making thecross-sectional areas of the legs which support the center span muchless than the area of the center span emr target area. In FIGS. 12(a)and 12(b) the center span 54a is rectangular, while a pair of supportlegs 56a extend from opposite sides of the center span and are quitenarrow where they meet the center span. The support legs broaden out tothe same width as the center span along the substrate to provide a moresecure bridge retention. By contrast, in FIGS. 13(a) and 13 (b) thecenter span 54b has the same rectangular configuration, but the supportlegs 56b consist of narrow tabs 56b at each corner of the center span.In FIGS. 14(a) and 14(b) the center span 56 has a generally serpentineconfiguration to increase its effective length, and thereby enhance itsthermal isolation, by increasing the length of the average thermal pathbetween the center span and the substrate. Support legs 56c consist ofnarrow tabs at opposite ends of the center span.

A thin metal layer, typically on the order of 100 Angstroms that is,0.01 micron thick, is established over each of the support legs toprovide an electrical shunt between the center span and the underlyingreadout circuit. This significantly reduces I² R heating in the legs.Because they are so thin, the metal layers themselves do not conductheat significantly.

FIG. 15 is a simplified schematic diagram of an output circuit that canbe used to monitor the bridge resistance of each detector cell, andthereby the level of emr received by that cell. Similar readout circuitswould be replicated for each of the individual cells. The circuitconsists of a basic voltage divider formed by the variable bridgeresistance R_(B) in series with a load resistor R_(L). Positive andnegative bias voltages V_(b) are applied across the series network,while the output at the midpoint of the two resistors is amplified by anamplifier A1 and delivered as an output voltage V_(o). Any emr-inducedchange in R_(B) will alter the bridge circuit, and adjust V_(o)accordingly.

A computer model has been devised to estimate the performance of a thinamorphous silicon bridge detector of this type. The resulting estimationfor the bridge responsivity to applied emr in a vacuum is 4×10³volts/watt, for an amorphous silicon bridge having a thermalconductivity of about 0.0256 J/cm/°K., a temperature coefficient of 2%,a resistivity of 100 ohm-cm, dimensions of 100×40×1 microns, and a biasvoltage V_(b) of 5 volts. Assuming Johnson (or resistive) noise is alimiting source of noise figure, the estimated noise equivalent power isnear 3.4×10³¹ 10 W, and the corresponding detectivity is 1.8×10⁸cm-Hz⁰.5 /W for an effective bandwidth of 100 Hz. The minimum resolvabletemperature difference at 300 ° K. background is then close to 0.17° C.,and the total power consumption for a 320×160 detector array is on theorder of 2 Watts. This estimated performance is quite suitable forapplications of an uncooled IR imaging system.

A more detailed schematic of an output circuit for each resistor bridgedetector is provided in FIG. 16. Resistors R1 and R2 are chosen tomaximize the product of the transistor transconductance and outputresistances. The upper transistor T2 provides a large dynamic load, butrelatively low static load, for the bottom amplifying transistor T1,resulting in a large gain.

Since the detector array will have a large number of output circuits ofthis type, one for each bridge detector, the output circuits arearranged in a matrix configuration with a strobe signal S applied to anoutput transistor T3 to obtain an output from each individual readoutcircuit at a desired strobe time. A capacitor C across the drain andgate of amplifier transistor T1 provides a large equivalent capacitance,and a corresponding low level of Johnson noise; the strobe readoutprocess via T3 also discharges (resets) capacitor C. The equivalentcapacitance of the circuit is given by the expression C(1+A), where A isthe amplifier gain.

A physical construction by which T1 and C can be monolithicallyintegrated into the same structure is shown in FIG. 17. A gatemetallization 58 is formed over an oxide layer 60 on the substrate, witha heavily doped semiconductor layer 62 underlying a portion ofmetallization 58 and oxide 60, and extending laterally to one side. Agate connection is made to metallization layer 58, a drain connection ismade to doped semiconductor layer 62, and a source connection is made toa heavily doped region 64 on the opposite side of oxide layer 60 fromdrain layer 62. The transistor channel comprises the portion of thesubstrate between drain 62 and source 64, while capacitor C isestablished by the aligned portions of gate metallization 58 and drainlayer 62.

While the provision of a large equivalent capacitance in the circuit ofFIG. 16 produces a significant reduction in Johnson noise, such noise isstill a limiting factor in the operation of the circuit. An alternatereadout circuit is shown in FIG. 18 which further reduces the Johnsonnoise level. This circuit employs a two-stage amplifier, with thecapacitor C connected across the second amplifier A2 and an inputresistor R2 for A2. Johnson noise is reduced by a factor of √R2/RL. Inaddition, the capacitance of C can be reduced, with a correspondingreduction in the chip area required by the capacitor, to the extent thetotal amplification of the two-stage amplifier of FIG. 18 exceeds theamplification of the single-stage amplifier discussed above.

FIG. 19 shows a different form of bridge emr detector which, as with theresistance bridge detector discussed above, can be dimensioned to detectvarious emr wavelengths. The sensor is basically a thermocouple bridge66 which generates a thermally induced voltage in response to receivedemr. It consists of a bridge structure with alternating layers ofdissimilar materials. While in theory any two different materials couldbe used to produce a thermal junction voltage, metal-semiconductorjunctions have generally been found to produce the highest voltagelevels. The bridge is preferably fabricated as a stacked layer, withthin metal layers 68 such as platinum alternating with semiconductorlayers 70 such as amorphous SiGe. The metal layers are preferably about0.1-0.2 microns thick, while the semiconductor layers are about 0.5-2microns thick to give sufficient mechanical strength. The thermocouplestack is surmounted by a heavily doped semiconductor back contact layer72, which in turn is capped by a layer 74 of radiation absorbingmaterial. The back contact layer 72 can be formed by a low energy ionimplant to a depth on the order of 0.1 microns. An opening of "via" 76is formed in the radiation absorbent coating 74 to provide a passage foran electrical lead line 78 to establish contact with back contact layer72. A readout circuit 89 is provided on the substrate, below and atleast partially shaded by the bridge 66, to monitor the emr-inducedthermocouple voltage.

A simplified schematic of a readout circuit for this embodiment is givenin FIG. 20. A load resistor R_(L) is connected in series with thethermocouple junction, represented as a variable voltage source 82. Abias voltage source 84 is connected across this voltage divider circuit,with the voltage V_(o) across R_(L) monitored as an indication of theemr-induced voltage across the junction.

The detector array of the present invention can also be implemented as aSchottky junction bridge 86, illustrated in FIG. 21. In this embodiment,a semiconductor layer 88 is formed over a bridge-shaped metal layer 90to establish a Schottky junction. The semiconductor layer 88 should havea small bandgap to obtain a high leakage current across the junction,with a corresponding high sensitivity. Semiconductors such as amorphousgermanium or amorphous tin are suitable for this purpose. A heavilydoped semiconductor that is substantially conductive can also be usedinstead of metal layer 90. As with the thermocouple embodiment, thebridge is tipped by a doped back contact layer 92 and a radiationabsorbent layer 94, with a lead 96 electrically contacting the backcontact through a via 98 in the radiation absorbent coating.

The readout circuit 100 for each bridge is again preferably locatedunder the bridge, and at least partially shaded thereby. A simplifiedform of a suitable readout circuit is shown in FIG. 22. The Schottkyjunction bridge, represented by Schottky diode 102, is connected inseries with a load resistor R_(L). A voltage source 104 is connectedacross the series circuit to reverse bias the Schottky junction. Changesin the Schottky leakage current appear as an output voltage V_(o) acrossR_(L), which is monitored as an indication of the emr incident on thebridge.

The detector bridges for each of the three embodiments, resistance,thermocouple and Schottky junction, are fabricated in a manner similarto the IR simulator bridge described previously. Spacer layers areestablished at the intended bridge locations, the bridges are formedover the spaces layers, and the spacer material is then dissolved oretched away.

An emr source 106 can be imaged onto an array 108 of the described emrdetectors by placing a collimating lens 110 in the path of the emrtravelling towards the detector array. While only a small number ofindividual detector bridges are shown on detector array 108 forsimplicity, in practice the arrays would have a large number ofindividual bridge detectors. The actual number for any particularapplication would depend upon the required bridge dimensions for theradiation to be detected, the available dimensions for the overallarray, and the desired resolution. An array of 320×160 individualdetector elements might by typical.

In practice, the sensitivity of the resistance of the bridge resistorand that of the load resistor to variations in their dimensions, as wellas variations in the output voltage of the pixel circuits due tononuniformities in bias voltage and amplifier gains, makes it importantto have an on-board compensation technique to correct for the resultingimage nonuniformities. Because the dimensional changes are small andcould in principle also be caused by mechanical and/or ambient thermalstresses, it is necessary to correct on a periodic bases. To correct forsuch nonuniformities, a shutter can be placed in front of the detectorarray and periodically closed to obtain a black readout. This readout isstored in a ROM or other convenient storage device, and used tocompensate readouts with the shutter open. The system operation ispreferably performed in an analog format, with the calibrationinformation stored digitally. With a video sample rate of 30 frames persecond, recalibration might be performed perhaps once an hour.

Electrical input and output lines can be conveniently run along thesubstrate, passing under the individual detector bridges to conservesubstrate area, in a manner similar to that shown in FIGS. 2 and 10 forthe IR simulator array. Power lines, ground lines, output lines andstrobe lines would typically be employed.

Since numerous variations and alternate embodiments will occur to thoseskilled in the art, it is intended that the invention be limited only interms of the appended claims.

We claim:
 1. A two-dimensional sensing array for electromagneticradiation (emr), comprising:a substrate, an array of emr detector cellsin said substrate, each of said cells including an emr sensitive bridgestructure spanning a portion of the cell, said bridge structure beingshaped to form a generally thermally insulative gap between the bridgestructure and substrate, and having a defined characteristic whichvaries in accordance with the amount of emr received by the bridge, andmeans for monitoring said defined characteristic for each of said cells.2. The emr sensing array of claim 1, said bridge structures comprisingrespective resistor bridges.
 3. The emr sensing array of claim 2, saidresistor bridges comprising a center span having a target area forreceiving emr and a plurality of support legs elevating the center spanabove the substrate, the geometries of said center span and support legsbeing selected to enhance emr absorption by the center span relative tothermal losses from the center span to the substrate through the supportlegs.
 4. The emr sensing array of claim 3, wherein the cross-sectionalareas of said support legs are substantially less than the areas oftheir respective target areas to thermally insulate the center span fromthe substrate.
 5. The emr sensing array of claim 4, wherein a pair ofsupport legs are provided for each center span, generally centered onopposite sides of the support span.
 6. The emr sensing array of claim 4,said center spans generally comprising rectangles, wherein a support legis provided adjacent each corner of said rectangles.
 7. The emr sensingarray of claim 3, said center spans having a generally serpentineconfiguration to increase their effective lengths and reduce thermalconduction losses therefrom.
 8. The emr sensing array of claim 2, saidmonitoring means comprising respective resistors connected in serieswith the resistor bridges to form respective voltage divider circuits,means for applying a voltage to said voltage divider circuits, and meansfor monitoring the voltages across said resistors as a function of theemr received by said resistor bridges.
 9. The emr sensing array of claim8, said monitoring means comprising respective monitoring circuits ateach cell, said monitoring circuits including multiple-stage amplifierreadout circuits to reduce Johnson noise associated with said voltagedivider circuits.
 10. The emr sensing array of claim 2, said monitoringmeans including respective monitoring circuits at each cell, saidresistor bridges comprising a center span having a target area forreceiving emr, a plurality of support legs elevating the center spanabove the substrate, and a thin layer of conductive material on saidsupport legs electrically connecting said center span to said monitoringcircuits, said layers of conductive material being thin enough tosubstantially maintain the thermal isolation of said center spans formthe substrate.
 11. The emr sensing array of claim 2, said resistorbridges being formed from an amorphous semiconductor material.
 12. Theemr sensing array of claim 1, said bridge structures comprisingrespective junction devices having an electrical characteristic whichvaries in accordance with the amount of emr received by the junctiondevice, and said monitoring means comprises means for monitoring saidelectrical characteristic for each of said cells.
 13. The emr sensingarray of claim 12, said junction devices comprising junctions of unlikematerials selected to generate a thermally induced voltage, saidjunction devices being configured to heat in response to applied emrwithin a desired range of wavelengths, and said monitoring meanscomprises means for sensing thermally induced voltages in each of saidjunction devices.
 14. The emr sensing array of claim 13, said junctiondevices comprising semiconductor-metal junctions.
 15. The emr sensingarray of claim 14, said junction devices comprising a stacked pluralityof alternating semiconductor and metal layers.
 16. The emr sensing arrayof claim 13, said monitoring means comprising respective resistorsconnected in series with said junction devices to form respectivevoltage divider circuits, means for applying a voltage to said voltagedivider circuits, and means for monitoring the voltages across saidresistors as a function of the emr received by said junction devices.17. The emr sensing array of claim 12, said junction devices comprisingSchottky diode bridge structures, and said monitoring means comprisesmeans for reverse biasing said Schottky diode bridge structures andmonitoring their leakage currents as a function of emr-induced heating.18. The emr sensing array of claim 17, said Schottky contact bridgestructures comprising adjacent layers of a semiconductor with a metal ordoped semiconductor conductor, said layers meeting along a Schottkycontact junction.
 19. The emr sensing array of claim 18, wherein saidsemiconductor is amorphous germanium or amorphous tin.
 20. The emrsensing array of claim 1, said emr sensitive bridge structures includinga layer of emr absorbing material for receiving emr and activating theremainder of the bridge structures to vary said defined characteristicin response to emr received by said absorbing material.
 21. The emrsensing array of claim 1, said monitoring means including a readoutcircuit for each cell situated on the substrate at least partially underthe bridge structure for that cell and at least partially shaded therebyfrom applied emr.
 22. The emr sensing array of claim 21, said monitoringmeans including actuating and monitoring lead lines extending along thesubstrate for respectively actuating the bridge structures response toreceived emr and monitoring said responses, at least some of said linesextending under at least some of said bridge structures.
 23. The emrsensing array of claim 1, further including a lens for imaging an emrsource onto said array.
 24. The emr sensing array of claim 1 whereinsaid means for monitoring further includes compensating meanscomprising:shutter means, placed in font of the detector arry forperiodic closure to obtain black readouts, and storage means positionedto receive said black readouts and provide compensation for subsequentreadouts with the shutter open.
 25. An emr detector, comprising:asubstrate, an emr bridge structure spanning a portion of said substrateand separated therefrom by a generally thermally insulative gap, saidbridge structure having a defined characteristic which varies inaccordance with the amount of emr which it receives, means for elevatingthe bridge structures above the substrate, and means for monitoring saiddefined characteristic.
 26. The emr detector of claim 25, said bridgestructure comprising a resistor bridge.
 27. The emr detector of claim26, said bridge structure and elevating means comprising a center spanhaving a target area for receiving emr and a plurality of support legselevating the center span above the substrate, the geometries of saidcenter span and support legs being selected to enhance emr absorption bythe center span relative to thermal losses from the center span to thesubstrate through the support legs.
 28. The emr detector of claim 27,wherein the cross-sectional areas of said support legs are substantiallyless than the target area of said target area to thermally insulate thecenter span from the substrate.
 29. The emr detector of claim 28,wherein a pair of support legs are provided for the center span,generally centered on opposite sides of the support span.
 30. The emrdetector of claim 28, said center span generally comprising a rectangle,wherein a support leg is provided adjacent each corner of saidrectangle.
 31. The emr detector of claim 27, said center span having agenerally serpentine configuration to increase its effective length andreduce thermal conduction losses therefrom.
 32. The emr detector ofclaim 27, said monitoring means including a monitoring circuit on thesubstrate, and further comprising a thin layer of conductive material onsaid support legs electrically connecting said center span to saidmonitoring circuit, said layers of conductive material being thin enoughto substantially maintain the thermal isolation of said center span fromthe substrate.
 33. The emr detector of claim 26, said monitoring meanscomprising a resistor connected in series with the resistor bridge toform a voltage divider circuit, means for applying a voltage to saidvoltage divider circuit, and means for monitoring the voltage acrosssaid resistor as a function of the emr received by the resistor bridge.34. The emr detector of claim 33, said monitoring means including amultiple-stage amplifier readout circuit to reduce Johnson noiseassociated with said voltage divider circuit.
 35. The emr detector ofclaim 26, said resistor bridge being formed from an amorphoussemiconductor material.
 36. The emr detector of claim 25, said bridgestructure comprising a junction device having an electricalcharacteristic which varies in accordance with the amount of emrreceived by the junction device, and said monitoring means comprisesmeans for monitoring said electrical characteristic.
 37. The emrdetector of claim 36, said junction device comprising a junction ofunlike materials selected to generate a thermally induced voltage andconfigured to heat in response to applied emr within a desired range ofwavelengths, and said monitoring means comprises means for a thermallyinduced voltage in said junction device.
 38. The emr detector of claim37, said junction device comprising a semiconductor-metal junction. 39.The emr detector of claim 38, said junction device comprising a stackedplurality of alternating semiconductor and metal layers.
 40. The emrdetector of claim 37, said monitoring means comprising a resistorconnected in series with said junction device to form a voltage dividercircuit, means for applying a voltage to said voltage divider circuit,and means for monitoring the voltage across said resistor as a functionof the emr received by the junction device.
 41. The emr detector ofclaim 36, said junction device comprising a Schottky diode bridgestructure, and said monitoring means comprises means for reverse biasingsaid Schottky diode bridge structure and monitoring its leakage currentas a function of emr-induced heating.
 42. The emr detector of claim 41,said Schottky contact bridge structure comprising adjacent layers of asemiconductor with a metal or doped semiconductor conductor, said layersmeeting along a Schottky contact junction.
 43. The emr detector of claim42, wherein said semiconductor is amorphous germanium or amorphous tin.44. The emr detector of claim 25, said emr sensitive bridge structureincluding a layer of emr absorbing material for receiving emr andactivating the remainder of the bridge structures to vary said definedcharacteristic in response to emr received by said absorbing material.45. The emr detector of claim 25, said monitoring means including areadout circuit situated on the substrate at least partially under saidbridge structure and at least partially shaded thereby from applied emr.46. The emr detector of claim 25 wherein said means for monitoringfurther includes compensating means comprising:shutter means, placed infront of the detector array for periodic closure to obtain blackreadouts, and storage means positioned to receive said black readoutsand provide compensation for subsequent readouts with the shutter open.